It is known that an aircraft, in particular a transport airplane, is generally provided with an ADC (<<Air Data Computer>>) type anemobarometric unit providing in real time a conventional CAS (<<Calibrated Air Speed>>) type speed. To do so, such anemobarometric unit is associated with total pressure probes (Pitot tubes) and it can be part for example of an ADIRS (<<Air Data Inertial Reference System>>) type air data and inertial data system representing an inertial reference unit integrating the functions of the anemobarometric unit.
Now, erroneous or absent air data (including airspeeds) can for example appear upon failures of systems, and erroneous information from sensors or upon the presence of frost or ice crystals.
It is known that frost is a meteorological phenomenon, the appearance conditions of which are related to humidity and static temperature. Frost may have some effects on the sensors, such as the total pressure probes, up to leading to a degradation of the measurement, making it sometimes erroneous (fixed or biased).
It is thus interesting to be able to have available on the aircraft an alternative airspeed adapted to be determined even is a case of a failure of an anemobarometric unit and/or associated pressure probes, including Pitot heads.
Moreover, it is known that it is possible to calculate a so-called aerodynamic airspeed from the current values of parameters (mass, load factor, incidence, etc.) related to the aircraft, including aerodynamic parameters (load factor, incidence, etc.)
However, the parameters being used to calculate such airspeed, being available on an aircraft, are not sufficiently precise to obtain an airspeed being sufficiently reliable and precise to be used aboard an aircraft.